Use of Track-End Alpha Particles from²⁴¹ Am to Study Radiosensitive Sites in CHO Cells

Monolayers of CHO cells placed on membrane filters were irradiated with alpha particles from a²⁴¹ Am source. Particle penetration into the cells was controlled by placing the cell sample at various distances from the source. Dosimetric and spectrometric measurements were performed at comparable positions using a parallel plate ionization chamber and a scintillation crystal spectrometer. Cell survival, as measured by conventional cloning techniques, was single hit in form. A pronounced minimum in mean lethal dose of 29 rad was observed for alpha particle beams that penetrated only about 3 μm into the cell. A pronounced maximum in inactivation cross section of$90\ \mu {\rm m}^{2}$, equal to about half the projected area of the nucleus, occurred for beams that penetrated only 5-7 μm into the cell. Thus, a single alpha particle penetration several micrometers within the cell nucleus was effective in killing the cell, while fully penetrating beams were actually less efficient; the latter beams required multiple particle traversals and about three times the cell dose to achieve the same effect. These results support the proposal that radiosensitive sites are located in a thin peripheral region of the nucleus.